Bootcamp showing incorrect free space

Hi I have a Macbook air with 256GB drive. In finder when I click Get info on Macintosh HD I see 90gb of free space. However when I go to bootcamp it only allows me to make a windows partition size of 20GB. Why is that so?

When you attempt to create a larger Windows partition does the Boot Camp utility warn you that there is not enough free space? If so, then the drive space has become fragmented. You'll need to defragment the drive and then run the utility again.